In the automobile industry, the various vehicle accessories such as power steering pump, oil and air pump, air conditioner, alternator, and the like, are operated by a single endless belt driven by a pulley connected to the engine crankshaft. Such system is typically referred to as a “serpentine” drive belt system. To provide optimum operating efficiency for the above-mentioned and other various accessories, it is imperative that a predetermined tensioning force be maintained on the drive belt to insure efficient performance of the accessories as well as satisfactory service life for the belt. Because of the length of the drive belt, there is a tendency for the belt to stretch due to wear and vibration, affecting the operating characteristics of the belt driven accessories. Therefore, it is conventional to use a belt tensioning device to provide a constant tensioning force on the belt to compensate for the increased length of the belt due to such stretching. Reference is made to commonly assigned U.S. Pat. Nos. 5,030,172; 5,443,424; 5,545,095; 5,718,649; 5,803,850 and 6,206,797, each of which is incorporated herein by reference, for examples of prior art belt tensioners.
A common type of belt tensioner embodies a stationary housing and an angular displaceable lever arm that carries a belt engaging pulley. A coil spring is braced against the stationary housing and displaceable lever arm and biases the latter toward the belt with a tensioning force varying in accordance with the vibrational nature of the belt. Despite the varying spring force, a substantially constant force acting upon the lever arm is maintained.
It is desirable that a belt tensioner be provided with a damping means to prevent excessive oscillation from occurring in the spring member. Such means is designed to absorb sudden shocks and to prevent a whipping action from occurring in the tensioner and drive belt. This damping means is especially critical when a coil spring is used for applying the belt tensioning force since it is inherent to coil springs to develop natural oscillation frequencies upon applying of the fluctuating counter force by the belt. Such fluctuations diminish the efficiency of tensioning force of the spring. However, the damping requirements are essential in order to enable the belt system to function over an extended period on a pulsating machine without affecting a tensioning force that acts upon the drive belt.
For example, a belt tensioning device has been proposed in U.S. Pat. No. 3,924,483 to Walker et al. wherein there is disclosed a torsional spring for pivotally moving one of the vehicle accessories to achieve the desired tensioning forces. Other tensioners of the above-described type utilize a pair of torsional springs for pivotally moving a lever and an idler pulley into belt tensioning engagement which results in an economic and compact unit. Specifically, in this type of tensioner, each spring is mounted on a respective side of the lever and engaged with the lever and housing for biasing the intervening lever toward the belt in a belt tensioning direction. Furthermore, the automobile industry has recognized the vibrational environment of an automobile belt system and its effect on spring oscillation.
U.S. Pat. No. 4,696,663 discloses a belt tensioner that includes a stationary housing 12, a lever arm 30, and a torsional spring 20 which is braced against the housing and the lever and biases the lever in a belt-tensioning direction. The belt tensioner is equipped with a brake 60 actuated by the spring into frictional engagement with a housing wall 13. Since the torsional spring provides both the tensioning force for the lever and the actuating brake force, the amount of damping is proportional to the belt tensioning force.
U.S. Pat. No. 4,473,362 discloses a separate damping body 108 whose damping characteristics are not constant but vary proportionately with the position of a pivot structure 40 relative to a stationary structure 36. A coil spring is mounted between the fixed and pivoted structures for resiliently biasing the latter in a direction away from the first limiting position thereof toward the second limiting position with a spring force, which increases as the pivot structure is displaced toward the belt. The damping body has a relatively tight fit at its inner periphery with the lower outer periphery of a core member 48 and a relatively loose fit between its exterior periphery and an interior periphery of the pivot structure. Angular displacement of the pivot structure between its first and second limiting positions is accompanied by a sliding movement between the exterior periphery of the damping body and the inner periphery of a mounting portion of the contacting surfaces varies in accordance with the position of the pivot structure, the amount of friction likely varies as well and, hence, the torsional force required to overcome the frictional force may also vary. Thus, the arm advantageously experiences a greater damping effect in a belt-releasing direction.
Commonly assigned U.S. Pat. No. 6,206,797 to Quintus discloses belt tensioner configurations that create damping by increasing friction between the spring bushing arm and spring case and between the armplate bushing, armplate and arm. The increase in friction is achieved by increasing the axial load on the tensioner components using springs or deflected armplates. While providing effective damping, this configuration also increases the loads on critical wear areas that are essential to maintaining the alignment of the pulley. Accordingly, it is desirable to have a structure which overcomes the above-discussed drawbacks.